Low carbon particle producing gas turbine combustor

ABSTRACT

A combustion system for a gas turbine engine provides a reaction limiting means for preventing the combustion process within the combustion chamber of the engine from proceeding towards completion by limiting the reaction rate or duration of the reaction process sufficiently such that a clean combustion gas is produced having substantially less carbon particles than for a combustor effective for essentially complete combustion. The invention also provides an efficient method of removing carbon deposits which do form by controlling the combustor shutdown technique in such a way as to cause the carbon to be oxidized. The preferred embodiment provides an aircraft emergency power unit having a high pressure air storage tank, an aviation fuel storage tank, and a combustor with a reaction limiting means which combusts pressurized air and aviation fuel in a fuel rich ratio to produce a motive combustion gas in a reaction limiting means.

This is a division of application Ser. No. 07/876,343, filed Apr. 30,1992, now U.S. Pat. No. 5,307,633.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a gas turbine combustor and a method foroperating such a combustor so that the exhaust products flowed to aturbine will be relatively free of carbon particles. More specifically,this invention provides a combustor for a stored energy subsystem usingaviation fuel and compressed air for burning a fuel rich mixture toyield a controlled, high temperature, high pressure gas wherebyapparatus and method are provided for preventing the reaction fromreaching completion.

2. Description of Related Art

Generally, an aircraft has one or more primary engines which providethrust for the aircraft, as well as pressurized bleed air for theenvironmental control systems. The primary engine also provides power todrive electric generators and hydraulic pumps, both of which arenecessary for powering instruments and flight control systems. Inaddition, many aircraft also have an auxiliary power engine to provideelectric and hydraulic power, as well as bleed air to the aircraft whenthe primary engines are not operating, for example when the aircraft ison the ground. The auxiliary power engine may also provide power tostart-the primary engines either on the ground or in flight. Both theprimary engine and the auxiliary power engine operate on aviation fuelfrom the aircraft's main fuel tanks, mixed with air drawn from theatmosphere as the combustion components. For maximum fuel efficiency,these engines operate in an air rich, fuel lean mode. In many instances,starting the auxiliary power engine requires an external power sourcesuch as a ground based start cart, a pressurized air tank, or anemergency power system. Since the auxiliary power engine is primarilydesigned to operate on the ground where the air is relatively dense, theauxiliary power engine may be incapable of operating at higherelevations, for example above 55,000 feet. It is therefore evident thatin many applications the auxiliary power engine would not be able torestart a failed primary engine above 55,000 feet, and in this eventthere would be no electrical or hydraulic power available. Also, sinceboth the primary engine and the auxiliary power engine operate on fuelfrom the main fuel tanks, if this fuel supply is depleted there will beno source of power for the electrical and hydraulic power systems toallow the pilot to control and land the aircraft.

It is therefore desirable to have on an aircraft an emergency powersystem capable of operating independent of external conditions which canprovide emergency electrical and hydraulic power to the flight controlsystems and may be used to restart the auxiliary or primary engines.These are the minimum requirements of the emergency power system. Sincethey are only operated in the event of an emergency, these systemsremain stored and inactive for long periods of time, but are required tostart instantly and provide continuous power output for a prespecifiedduty cycle. Ideally, such an emergency power system would be compact,lightweight, highly reliable, easily maintained, require no specialhandling of materials or fuels, while providing a combustion processwhich is controllable and which produces a clean, combustion gas.Presently, emergency power units primarily rely on liquid hydrazinebased fuels sprayed into a catalyst bed to generate a pressurized gas.These units are in use on several aircraft and combine high performancewith low weight.

However, liquid hydrazine is highly corrosive and toxic, therebyrequiring special handling procedures and design considerations. Thecatalyst material is expensive, and when the catalyst is depleted itmust be replaced. Further, the combustion gas which is produced is toxicand therefore limits ground testing of the emergency power unit.

To overcome such problems emergency power systems were designed tooperate on a fuel rich mixture of aviation fuel and air which optimizesthe advantages of such a system to yield an emergency power system withthe improved characteristics of relying on an energy source which isreadily available, non-toxic, and clean burning, packaged in a compact,lightweight, highly reliable, and easily maintained emergency powerunit.

Such an emergency power system is disclosed in U.S. Pat. No. 4,777,793entitled "Emergency power unit" and its divisional U.S. Pat. Nos.4,934,136 entitled "Method of Operating an Emergency Power Unit" and4,898,000 entitled "Combustor for an Emergency Power Unit" assigned tothe present assignee.

A problem with using carbon based jet fuels such as contemplated for thepresent invention is that in a fuel rich environment formation of solidparticles occur, generally comprised of carbon, suspended in thecombustion product gas flow. These particles pose a significant problemeffecting the gas turbine's operation, durability, and reliabilitybecause it can cause erosion of gas nozzles and turbine blades as wellas clog gas passages downstream of the combustor.

SUMMARY OF THE INVENTION

The present invention provides a combustor and method for burningaviation fuel in a gas turbine engine such as an emergency power systemoperating on a fuel rich mixture of aviation fuel and air, tosignificantly reduce or substantially prevent the formation of solidparticles in the combustion product gas flow.

Briefly, the invention provides a gas turbine engine combustor for usein a fuel rich combustor environment and methods for limiting the degreeto which the combustion reaction proceeds towards completion. Fourparticular techniques including apparatuses and methods are contemplatedby the present invention which individually and in combination may beused to limit the degree to which the combustion reaction proceedstowards completion.

The first technique reduces the characteristic length of the combustor,the second decreases the combustion gas temperature, the third decreasesthe combustion pressure, and the fourth increases the fuel droplet sizerelative to those values generally used for complete combustion.

The invention also presents a method of removing carbon deposits oncethey have formed. This method consists of oxidizing the carbon byshutting off the flow of fuel prior to shutting off the flow of air whenterminating combustor operation.

The present invention provides a light weight gas turbine combustor foroperation in a fuel rich environment such as in an aircraft EPU thatsubstantially reduces the formation of solid particles in the combustionproduct gas flow. Another advantage of the present invention is that itcan be easily adapted for use with existing combustors and EPU systems.

These and other advantages of the present invention are specifically setforth in, or will become apparent from the following detaileddescription of a preferred embodiment of the invention when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of the invention are explainedin the following description, taken in connection with the accompanyingdrawing where:

FIG. 1 is a schematic diagram of the emergency power unit embodying theprinciples of the present invention.

FIG. 2 is a partially schematic partially cross-sectional view of theemergency power unit combustor.

FIG. 3A is a diagrammatic cross-sectional view of an emergency powerunit combustor illustrating the prior art.

FIG. 3B is a diagrammatic cross-sectional view of an emergency powerunit combustor illustrating one embodiment of the present invention.

FIG. 4 is a partially cross-sectional view of the fuel nozzle of thecombustor of the emergency power unit.

DETAILED DESCRIPTION OF THE INVENTION

The present invention illustrated herein provides a means, both methodsand apparatuses, to prevent the combustion process of a gas turbineengine combustor, preferably operating with a fuel rich mixture, fromproceeding towards completion by limiting at least one combustorreaction rate parameter, either the duration or speed of the reactionprocess or both, sufficiently such that a clean combustion gas isproduced having substantially less carbon particles than are produced ina combustor designed for essentially complete combustion. One way tomeasure cleanliness of the combustion process with respect to carbonparticles is to measure its opacity. A substantially clean combustiongas should have an opacity of about 20% or lower and combustorsoperating in a regime with opacity much greater than 20% results inexcessive carbon formation as an exhaust product.

Illustrated in FIG. 1 is an emergency power unit 20 having a highpressure air tank 22 and an aviation fuel tank 24 as stored energymeans. Air tank 22 is connected via a high pressure air line 28, an airpressure regulator 36, and a regulated pressure line 29 to a combustor26. Air line 28 also includes an air shutoff valve 32, while regulatedpressure air line 29 includes an air temperature sensor 38 and an airflow control valve 40. Similarly, fuel line 30 between fuel tank 24 andcombustor 26 includes a fuel shutoff valve 42, and a fuel control valve46. An electrical controller 50 for emergency power unit 20 iselectrically connected to air shutoff valve 32, air temperature sensor38, airflow control valve 40, fuel shutoff valve 42, fuel control valve46, as well as being electrically connected to an igniter 68, a pair ofcombustor temperature sensors 58, and a turbine speed sensor 56. Turbinespeed sensor 56 is in relative proximity to, and senses the speed of anoutput shaft 54 which is attached to a turbine 52 which is driven by thecombustion gases produced within combustor 26. Output shaft 54 fromturbine 52 is connected to a gearbox 70 to drive a generator 72 and anhydraulic pump 74. Downstream of turbine 52, the combustion gases areexhausted through an exhaust duct 60.

Prior to operation, high pressure air tank 22 is filled to severalthousand p.s.i. with compressed air and fuel tank 24 is filled withaviation fuel. Fuel tank 24 may be a positive displacement pressurizedpiston or bladder type of fuel tank. When emergency power is required,controller 50 opens air shutoff valve 32 and fuel shutoff valve 42 tostart the flow of air and fuel through respective air line 28 and fuelline 30. Subsequently, air coming from high pressure air tank 22 isdelivered to pressure regulator 36 which reduces the air pressure to anappropriate regulated pressure level controlled by controller 50.

Controller 50 also uses the air temperature signal from air temperaturesensor 38 to direct air flow control valve 40 and fuel control valve 46to deliver a precise fuel to air ratio to combustor 26 at desiredpressures. The fuel and air mix within combustor 26 and igniter 68commences the combustion process. Once initiated, combustion isself-sustaining as long as fuel and air are delivered to the combustor.

Controller 50 powers and controls igniters 68, and receives input backfrom combustor temperature sensors 58, and turbine speed sensors 56 tocombine with the air and fuel temperature measurements in order tooptimize the performance and efficiency of the emergency power system,while regulating the turbine speed to a nominal, normally very highspeed by adjusting fuel and air flow rates to combustor 26.

It may be appreciated that as the compressed air supply within highpressure air tank 22 is depleted, the temperature of the remaining airdrops rapidly. Thus, controller 50 must continuously monitor theregulated air temperature and the fuel temperature, which is alsosubject to variation, in order to properly modulate the fuel to airratio delivered to combustor 26.

The desired air to fuel ratio is in the fuel rich range of between 1.4:1and 3.8:1 by weight to produce a combustion gas having a temperaturebetween 1100° and 2000° F. The optimum air to fuel ratio for the presentsystem is approximately 2.9:1, producing a combustion gas having atemperature of 1650° F. The combustor 26 illustrated herein is designedto operate with good controllability throughout a chamber pressure rangeof 700 to 140 psia. Temperature and pressures are maintained bycontroller 50 as explained above. Prior designs were made to operate atan air to fuel ratio of 3.5:1 to produce a desired combustion gastemperature of 1850° F.

Combustor 26 of FIG. 1 is shown in a more detailed cross-sectional viewin FIG. 2. A combustion chamber 251 having a conical section 252 isenclosed by a generally cylindrically shaped thermal liner 253 and aconical liner 254 which tapers down at its exit 256, having a circularexit area A, to an attachment 267 of a nozzle box 261. Thermal liner 253is wrapped in ceramic insulation 255 and enclosed by a pressure shell257. The volume of combustor 26 is generally defined by combustionchamber 251. The characteristic length of a combustor such as combustor26 is defined as the volume divided by its exit area which is A in theillustration of FIG. 2.

The present invention provides for a means for limiting the degree towhich the combustion reaction in combustion chamber 251 proceeds towardcompletion. One technique for doing so is to limit the duration of thereaction process. The duration is often referred to as the residencetime of the reaction, which is the average time reactants are within thecombustion chamber. Residence time can be reduced by reducing the volumeof a combustion chamber relative to its exit area or conversely asillustrated herein increasing the exit area relative to the combustionchamber volume. The volume of a combustion chamber divided by the areaof its exit nozzle is referred to as the characteristic length of thecombustor L*. By reducing the characteristic length L* of a fuel-richcombustor sufficiently to prevent the combustion reaction from reachingcompletion then, the gas cleanliness of the combustion gases can beimproved.

FIGS. 3A and 3B illustrates a means by which L* of combustor 26 can bereduced without decreasing cylindrical diameter D and length L ofcombustion 251. This is a highly desirable embodiment of the presentinvention for retrofitting or modifying existing designs, particularlyfor existing combustors for which operating conditions are well known. Aprior art combustor, i.e. one that permits the combustion's reaction tosubstantially proceed to completion, is illustrated in FIG. 3A. FIG. 3Billustrates how, by increasing the exit area without proportionally thevolume of combustion chamber 251, L* may be substantially decreased suchas by a factor of 3.

By way of example exit area A₂ of FIG. 3B is three times larger than theexit area A₁ in FIG. 3A, the diameter and radius R₂ of exit area A₂ are1.788 times as large as the diameter and radius R₁ of exit area A₁, andtherefore L* is one third as long.

Referring back to FIG. 2, the opposite end of combustion chamber 251 isenclosed by an injector head 231 which has attached thereto a plenumcover 229 and an injector assembly 227. Attached to plenum cover 229 isan air inlet assembly 225 which delivers air to an outer air plenum 233enclosed by plenum cover 229. A plurality of air passages 237 ininjector head 231 deliver air from outer air plenum 233 to an inner airplenum 235. An air swirler 239 mounted to injector head 231 includes aplurality of vanes 241 deflecting the air as it passes from inner airplenum 235 into combustion chamber 251. Injector assembly 227 of FIG. 2as shown in FIG. 4 includes a nozzle body 273, a tangential poppet 275,a filter screen 277, a compression spring 279 and a cylindrical insert281.

Combustor 26 is specifically designed to promote the mixing of fuel andair as it enters combustion chamber 251, ignite the mixture, andmaintain the combustion process for extended periods of time withoutexcessive carbon deposit buildup or melting of the combustor body.Aviation fuel flows to injector assembly 227 through the center ofinsert 281 within nozzle body 273 and is strained by filter screen 277before flowing around a tangential poppet 275 held in place by wirecompression spring 279. The aviation fuel flows out through an opening274 at the end Of nozzle body 273. Simultaneously, air flows through airinlet 225 into outer air plenum 233 where it is distributedcircumferentially about injector assembly 227 before flowing through airpassages 237 into inner air plenum 235 whereupon vanes 241 of airswirler 239 deflect the air and fuel mixture as it enters combustionchamber 251.

One technique contemplated by the present invention to improve gascleanliness is to increase the size of the fuel spray droplets producedby injector assembly 227. Larger droplets have less surface area for agiven volume of fuel. The smaller surface area increases the timerequired to vaporize the fuel for the combustion process. Properlydesigning the size of opening 274 using well known analytical,empirical, or semi-empirical techniques can provide a means for limitingthe degree to which the combustion reaction in combustion chamber 251proceeds toward completion by reducing the reaction rate of thecombustion process.

Injector assembly 227 can be designed such as by sizing opening 274 toproduce fuel droplet sizes larger than those for which would be producedfor a combustion process which proceeds essentially to completion and issufficiently large enough to reduce the reaction rate of the combustionprocess so that the process is sufficiently prevented from proceedingtowards completion so that it produces substantially less carbonparticles.

A spark from igniter 68 commences the combustion process. The thermalliner 253 rapidly heats up to approximately the combustion temperature.Ceramic insulation 255 prevents the conduction of heat to pressure shell257 such that during the limited duration of normal operation thetemperature of pressure shell 257 does not exceed 500° F. while thermalliner 253 may obtain temperatures up to 1800° F. The combustion gasesthen flow out of combustor 26 into nozzle box 261. Nozzle box 261distributes the combustion gases through one or more nozzles 263 toimpinge upon turbine 52, and provide a motive force to turbine 52causing turbine 52 to rotate about output shaft 54 and thus drivegenerator 72 and/or hydraulic pump 74.

The unique design of the walls of combustor 26 promotes and stabilizesthe combustion flame while minimizing the weight of the combustorsystem. Thermal liner 253 which is made of Inconel steel and has a verythin cross-section, rapidly heats up to the temperature of the combustorflame. Once thermal liner 253 has heated sufficiently hot thermal liner253 tends to stabilize the combustion process which is self-sustaining.Ceramic insulation 255, which is made of woven ceramic cloth and ceramicfiber mat, prevents the conduction of heat to pressure shell 257, whichis also made of Inconel steel, and thereby minimizes the thickness ofthe cross-section of pressure shell 257 which is required to contain thecombustion pressures. A further benefit of operating thermal liner 253at combustion temperatures is that it effectively prevents the buildupof carbon deposits on the walls of combustion chamber 251.

Another technique is to decrease the combustion chamber pressure. Lowerchamber pressures result in lower gas densities and less molecularinteraction between gases. This reduction in molecular interactiondecreases the rate of the chemical reactions, this limiting the degreeof completion of the reaction and improving gas cleanliness.

Each of these factors limits the degree to which the reaction is carriedto completion, and thus decreases the amount of carbon formed.

Clean burning fuel-rich combustors can be used to generate combustiongases to drive turbines in systems which produce some combination ofpneumatic, electric, hydraulic or mechanical power; the combustion gasescan be used directly to generate thrust or pneumatic power; or thefuel-rich combustion gases can be used as a fuel, for example as a fuelfor the second stage in a two-stage combustion process.

Applications for clean-burning fuel-rich combination include aircraftsystems, systems for other vehicles and stationary systems. Specificexamples of applications which are currently under consideration includeaircraft main engine start systems, aircraft auxiliary power unit startsystems, and aircraft emergency power systems.

The present invention provides clean-burning fuel-rich combustionsystems that can be adapted for use with any hydrocarbon fuel. Thepresent invention contemplates the use of fuels other than jet fuel asillustrated herein. The fuel can be supplied from a dedicated fuelsystem or, alternately, from any available fuel supply. A dedicated fuelsystem might consist of a gas-pressurized fuel tank using free-surfaceexpulsion, or a bladder or piston expulsion device. An aircraft'sprimary fuel system is an example of an available fuel supply whichmight alternately be used. A fuel pump may be used to obtain the desiredcombustor fuel inlet pressure when using these sources of fuel.

In order to increase the number of starts that can be made betweencleaning intervals with a JP fuel and air combustion system such as inthe present invention, a combustor shutdown technique is used thatcleans the turbine inlet nozzles 263 shown in FIG. 2 after each run.Referring to FIG. 1, upon system shutdown, the preferred embodiment usescontroller 50 to signal fuel shutoff valve 42 to shutoff the fuel flowwhile air shutoff valve 32 remains open. With this shutdown sequence,the high temperatures of combustor 26 that exist after operating thecombustor, together with the oxygen available in the air flow acttogether to oxidize the carbon that is built up around nozzle inlets263. This substantially allows the total nozzle area to be restoredafter each run and the unit's maintenance cleaning interval issignificantly increased.

While a number of specific embodiments of the present inventionpresented herein have been described fully in order to explain theinvention's principles, it is understood that the invention is not to beconstrued as being limited thereto and that various modifications oralterations may be made to the described embodiments without departingfrom the scope of the invention as set forth in the appended claims.

We claim:
 1. A combustion system for a gas turbine having turbine nozzleinlets comprising:a combustor for burning fuel with an oxidizer, a fuelvalve effective for turning on and shutting off fuel to said combustor,an oxidizer valve effective for turning on and shutting off oxidizer tosaid combustor, a combustor controller effective for controlling flow offuel and oxidizer to said combustor and programmed to shutoff the fuelflow to said combustor upon combustor system shutdown while oxidizer isallowed to continue to flow for a sufficient enough time to allow carbondeposits in a turbine inlet of the gas turbine to be oxidized.
 2. Amethod for operating a gas turbine combustor comprising the followingsequential steps:A) running fuel and an oxidizer for the fuel underpressure to the combustor and combusting the fuel and oxidizer agent inthe combustor, B) shutting off the fuel flow to the combustor uponcombustor system shutdown, and C) continuing to flow oxidizer for anamount of time sufficient to allow carbon deposits in a turbine inlet ofthe gas turbine to be oxidized.
 3. A method for operating a gas turbinecombustor as claimed in claim 2 wherein step A includes combusting thefuel and oxidizer agent in a fuel rich environment in the combustorwhich is effective for preventing the combustion process within thecombustor from proceeding towards completion by limiting at least onecombustor reaction rate parameter of a set of combustor reaction rateparameters sufficiently such that a clean combustion gas is producedhaving substantially less carbon particles than for a combustoreffective for essentially complete combustion, said set of parameterscomprising the duration of the reaction process and the speed of thereaction process.